"There is no rational way to lay the blame for the plane being shot down on the US or Trump"
That's only because you are completely and hopelessly IRRATIONAL; you have been conditioned to view anything other than the complete idiocy pouring out the piehole of your cult leader as IRRATIONAL.
Your blanket support for Trump, who deals with things beyond his comprehension in a completely irrational manner, proves this. I don't really have to.
Legally speaking, as far as the plane downing, Iran is fully responsible for the CIVIL liability that has been incurred from the unintentional result of what they deemed an act of self defense, but overall, the Trump administration shoulders a portion of the criminal liability for putting Iran into a crisis of self-defense inside their own home (i.e. their own country). If Trump was an individual in this country, and had plainly shown intent to cause grievous harm to someone inside their own home, then he could be held criminally liable for any casualty that was incurred during the said homeowner's act of self defense.
I understand as a boot-licking lackey of Trump you do not find the law rational. It is simply something to be trampled on whenever necessary.